There are several proposed models that offer appealing but incomplete
scenarios for the origin and evolution of galactic activity. Examples
include accretion flows, interactions and mergers, gas motions in
nonaxisymmetric potentials, and periodic ejection followed by infall
(hot gusts). Taking the observations and models as a whole and
exercising the prerogative of authors of Annual Review articles
to end their papers in unmitigated speculation, we now attempt to develop a
coherent view of the origin of galactic activity and, hence, to point
out additional areas for future research.

In the conventional picture two necessary (and perhaps sufficient)
conditions for the onset of nuclear activity are the presence of a
nuclear "monster" and a supply of "food"
(Gunn 1979).
The monsters are
presumed to be long-lived, compact, and presumably massive objects in
galactic nuclei.

Do all galaxies harbor monsters? We would argue that the lack of
activity in the nuclei of late-type galaxies, despite the apparent
abundance of food (as evidenced by the on-going star formation), means
that monsters do not live in such galaxies. Instead, the striking
incidence of activity in bulge-dominated systems strongly indicates
that the core of a bulge is the monster's favored lair and that
monsters are fairly common in systems with massive bulges. Moreover,
monsters need not all be alike. Some galaxies may have multiple
monsters in orbit about one another whose orbital axis can be an
important preferred direction.

Activity is more common in luminous (massive) galaxies, although the
dependence is stronger for radio sources than for emission-line nuclei
(i.e. Seyfert galaxies and quasars). The form of this dependence
suggests that monsters are fed more continuously in the more massive
galaxies. Dominant-cluster galaxies would be expected to receive an
almost continual supply of food from accretion flows or
cannibalization of cluster galaxies. Less massive galaxies may erupt
intermittently
(Bailey & Clube 1978),
perhaps suggesting that food for
the monster is only occasionally plentiful. Yet, low-level activity is
present even between outbursts in at least a third of all early-type
galaxies, so the monsters must get frequent snacks.

The role of galaxy rotation in feeding or fostering a monster needs
clarification; despite the naive expectation that food can be more
easily supplied to the nucleus in a low angular momentum system, the
most menacing monsters are seemingly found in galaxies in which
rotation is dynamically important. Perhaps a disk component, even
though weak, is required to supply food at a large rate. Better
statistical data on radio luminosity and galaxy rotation are needed.

The evidence seems to lend support to a number of currently popular
schemes to feed the monster. Mergers and interactions may be generally
relevant to nuclear activity, since they can obviously supply the
monster with fresh food in the form of gas
(Gunn 1979)
or, if
digestible, stars sent inward on plunging orbits. Interactions may
also trigger the growth of oval distortions in the galaxy's
gravitational potential leading, eventually, to the deposition of gas
at the nucleus
(Simkin et al. 1980).
Finally ellipticals may
periodically accrete their own ejecta from previous hot gusts, a
process which conceivably could be aided by the "back-pressure"
supplied by the gaseous medium found in compact groups of galaxies.

This description of the care and feeding of galactic activity
carries with it the usual obligatory statement "More and better
observations are needed." Few of the observational underpinnings of
this scenario are based on proper, statistically secure results. Many
facets of the problem are complicated by its multivariate nature, and
the truly important independent variables are yet to be identified.